42 



THE CELL 



G. MOTILITY 



It has already been necessary to refer briefly to the movements of the ele- 

 mentary organisms,, but since motility is one of the most important functions 

 of living substance, we must study it here in its different manifestations. 



1. The protoplasm of a plant cell inclosed within the membrane exhibits 

 different forms of motility. Some of these, as for example the migration of 

 chlorophyll bodies, take place very slowly (Fig. 25). In diffuse daylight the 



chlorophyll bodies are so placed that they pre- 

 sent their greatest surface to the light (T) ; 

 in direct sunlight they are so placed that their 

 narrow edge is turned toward the incident rays 

 (8) ; while in windows a third position (N) 

 may be taken. The purpose of these move- 

 ments is doubtless to protect the plant in 

 strong illumination from a too intense effect 

 of the light, and in moderate illumination to 

 secure the plant as great an effect as possible. 

 We observe in plant cells also streamings of 

 protoplasm which can be followed by the mi- 

 gration of the granules. In these movements 

 the protoplasmic particles either flow in dif- 

 ferent directions, often in great confusion 

 (circulation), or the protoplasm collected 

 along the wall is caught in a rotatory move- 

 ment all in the same direction, in which the 

 nucleus and often the chlorophyll bodies are 

 dragged along (rotation) (Fig. 16). 



2. The simplest kind of protoplasmic move- 

 ment in the naked cells proceeds in a manner 

 similar to that just mentioned, as may be ob- 

 served in the Amoebae (Figs. 20, 21, 26), and 

 in the leucocytes of multicellular animals 

 (Figs. 22, 27). During rest the Amoeba is 

 spherical. When it begins to move one or 

 more processes protrude from the periphery of 

 its body. By a kind of streaming movement 

 the protoplasm of the cell body then flows into 

 this process or processes and the position of the 

 entire mass of the animalcule is thereby changed. The protrusions are not 

 preformed structures, for the cell has the power to put out such a process 

 from any point of its surface and to withdraw it again. It is on account 

 of their transitory character that they are called pseudopodia, or false feet. 



The appearance of the pseudopodia in different species of elementary organ- 

 isms is very different. In unicellular animals provided with an external skeleton 

 they are modified according to the character of the openings in the skeleton 

 through which they protrude (cf. e.g., Fig. 14). We meet with short and thick, 

 or long and slender, threadlike or thorn-shaped pseudopodia ; or again with those 



FIG. 25. Varying positions of the 

 chlorophyll bodies in the cells 

 of Lemna triscula, according to 

 the direction of the incident 

 light rays, after Stahl. 



